1. Field of the Invention
The present invention relates to an AC-DC converter that receives an alternating-current power supply and outputs a direct-current voltage and, in particular, to a PFC (power factor correction) converter that suppresses harmonic current.
2. Description of the Related Art
Electric apparatuses connected to a commercial power supply are subjected to harmonic current restriction corresponding to their power capacity. Switching power supply devices that receive a commercial power supply typically include a PFC converter in order to satisfy such a restriction.
A general switching power supply device using a commercial alternating-current power supply as an input power supply rectifies and smoothes the commercial alternating-current power supply to convert it into a direct-current voltage and then switches the direct-current voltage in a DC-DC converter. Thus, the input current is discontinuous and significantly deviates from a sine wave. This results in harmonic current.
For this reason, a PFC converter is disposed between a full-wave rectifier circuit and a smoothing circuit including a smoothing capacitor in order to suppress the harmonic current.
This PFC converter includes a chopping circuit and operates so that the input current waveform has a sinusoidal shape similar to and in phase with the input voltage waveform. Thus, harmonic current is suppressed to a certain level or below, and the power factor is improved.
Generally known control theories include P (proportional) control, PI (proportional-integral) control, and PID (proportional-integral-differential) control. PFC converters generally use P control, since responsiveness is more important than the steady-state characteristics of the output voltage.
Examples of PFC converters that perform P control include Japanese Unexamined Patent Application Publication No. 07-87744. FIG. 1 is a circuit diagram of a PFC converter described in Japanese Unexamined Patent Application Publication No. 07-87744. Referring to FIG. 1, the PFC converter of Japanese Unexamined Patent Application Publication No. 07-87744 will be described.
In FIG. 1, a step-up voltage converter is provided. This converter obtains a voltage vr by rectifying an alternating-current voltage va of a commercial power supply using a rectifier circuit 1, provides the voltage vr to a reactor 2, interrupts current passing through the reactor 2 using a switching transistor 3, and extracts a voltage generated at the reactor 2 during the current interruption via a diode 4 as an output voltage vo, as well as smoothes and stabilizes the extracted voltage using a capacitor 5.
A value vo detected by a voltage divider 6, of the output voltage vo is provided to an error amplification circuit 7, and an error voltage ve indicating the difference between the detected value vo and a set value vs therefor is output. A multiplication circuit 8 receives the error voltage ve and the rectified voltage vr, multiplies both voltages, and outputs a voltage error signal Se, which is proportional to the error voltage ve and has the same pulsing waveform as that of the rectified voltage vr.
Current passing during the on-time of the switching transistor 3 and the waveform thereof are detected by a detection resistor 9, and this current waveform signal, Sc, and the above-mentioned voltage error signal Se are provided to a current error detection circuit 10 so that a current error signal S1 representing the waveform difference between both signals is output to the non-inverted input of a comparator 20. The comparator 20 compares the current error signal S1 with a sawtooth-shaped wave period signal so, which is received from a high-frequency oscillation circuit 21 and specifies the period during which the switching transistor 3 interrupts the current, and outputs an on/off instruction signal Sw, which is a PWM signal, to the switching transistor 3. Thus, the current passing through the reactor 2 is interrupted at a duty ratio specified by this on/off instruction Sw.
The PFC converter described in Japanese Unexamined Patent Application Publication No. 07-87744 obtains a high but finite gain in a low-frequency range. That is, even in a stable state, an error is present. As the output voltage error ve shown in FIG. 1 increases, the difference between the output voltage vo and the desired voltage vs increases, thereby reducing the output voltage.
In the PFC converter described in Japanese Unexamined Patent Application Publication No. 2007-129849, the gain is infinite in a direct current, such that the error can be eliminated in a stable state. However, in a transient state, such as an abrupt change in load, it takes time to charge or discharge the capacitor, such that the time taken until the output voltage settles is greater than that in the P-control PFC converter described in FIG. 1.